Sharp micro-scale tips are used in many electronic devices. For example, in imaging devices such as scanning tunneling microscopes and atomic force microscopes, in data storage devices that sense a data bit by a contact between the tip and a data storage media or by an electron beam emitted by the tip to sense the data bit stored in the media, and in display devices where the tip (e.g. a Spindt emitter tip) is used to emit an electron beam that excites photons from a phosphor layer.
For data storage devices, the tip is usually placed on a cantilever beam to allow the tip to be moved relative to the device or relative to the media that stores the data. Subtractive and additive tip processes can be used to form the tip. Prior subtractive tip processes include forming the tip on a cantilever as part of a silicon on insulator (SOI) wafer process. The tips are formed from a layer of single crystal silicon (Si) already on the wafer as part of the SOI process. Silicon etching and high temperature oxidation processes are used to form the tips. However, a disadvantage to the prior subtractive processes is that the high temperatures required to form the tip greatly limits an ability to integrate the tips and cantilevers with microelectronic devices on the same chip because the high temperatures can damage the microelectronic devices or a data storage media carried by the chip.
For display devices, such as field emission displays (FED) based on a Spindt type emitter, prior additive processes can be used to directly deposit a material for the tip through a circular mask. However, the prior Spindt tip processes require complex masking steps to form small holes and special deposition equipment such as an evaporator with an extremely long throw. Notwithstanding the use of correct processing techniques and equipment, getting good uniformity among many tips across the same wafer is very difficult.
Other prior additive tip processes exist to make tips. Typically, in those prior processes a small square or circle is patterned in a layer of a photoresist material and the pattern is used as a mask for a subsequent isotropic etch process. The mask is also referred to as a “hat” and the hat sits on top of a vertex of the tip as the tip is formed by the isotropic etch process. The tip gradually sharpens over time as the isotropic etch process proceeds. However, difficulty arises as the etch time for the isotropic etch process increases and the tip is sharpened to a sharp point where a surface area connecting the hat to the tip is very small and the hat is released from the tip. Hat release can damage the tip (i.e. breaking the sharp point) and/or cause a defect when the hat lands on some portion of the device. Consequently, one disadvantage of the prior additive tip processes is that the hat is supported by the vertex and can cause a defect and/or can damage the tip upon release. Moreover, the exposed tip may be damaged unless the isotropic etch process is stopped exactly as the hat is released. Therefore, another disadvantage of prior additive tip processes is that the tip is prone to damage unless an endpoint of the etching process is exactly timed for all of the tips being formed.
Consequently, there exists a need for a method of fabricating a sharp protrusion that can be accomplished at low processing temperatures using standard microelectronics processing equipment. There is also a need for a method of fabricating a sharp protrusion that eliminates damage and/or defects caused by hat release. Moreover, there is a need for a method of fabricating a sharp protrusion that eliminates the need to exactly time an etching endpoint and produces sharp protrusions having a uniform shape across a substrate they are fabricated on.